There are problems in the design of a tamper device for security sensors and for example, a round-shaped security sensor, such as a smoke detector, heat sensor or carbon monoxide sensor. Typically, a miniature switch is used to detect if the sensor has been opened. This switch has to activate when the body of the sensor is inserted and rotated into the mounting base to secure it and de-activate upon opening the sensor. After a few years, when the sensor is disassembled, the switch can fail to release and not annunciate that a tamper attempt has occurred. This could be due to, for example, vacuum, friction, spring, or material distortion. Known are sensor devices that include switches that have soft tops. However, in these designs, friction can cause distortion of the soft top when rotating the sensor during installation, and thus, a separate actuator is needed. The separate actuator complicates the design and additionally, the actuator is susceptible to establishing a “set position” (a normal position caused by material deformation, not by design) so that it may not release and as a result may cause a malfunction after a long duration of non-use.
A problem with conventional switches in known sensor devices is that large areas of a circuit board are required to be unpopulated, and complexity in the printed circuit board cover's topology, which increases size and cost and lessens product reliability.
Other known sensor devices require careful alignment of locating slots and tabs, with visual cues being required to complete the installation. One device requires aligning two tabs into slots, positioning the tabs, and then rotating part of the sensor device. In this case, if alignment is incorrect, the sensor could be damaged. Another known design for a sensor device to indicate tampering includes a switch mounted on a circuit board. The pressure of an actuating boss surface on the mounting plate retains the switch in an activated state. A common problem with this type of device is that pressure sensitive switches have a tendency to freeze in the closed position after being subject to being in the closed position for a length of time. This is due to an effect within the switch caused by a vacuum being formed with the internal disc-spring, or due to materials taking a “set position”, caused by the perpetually closed position. These switches are designed to work properly when normally open and occasionally closed, whereas tamper functions require the switch to perform the opposite of this.
When installing circular packaged sensors, such as a carbon monoxide, smoke or heat detectors that are permanently affixed to the wall or ceiling, the sensor is initially installed by a security system installer, and is removed from time to time by an end-user for battery replenishment. A disadvantage of current sensors with tamper indicating mechanisms is that it is difficult to replace the sensor to the mounting base after the necessary service has been completed. Previous products required aligning two tabs into slots and very carefully guiding them into position and then rotating them. If alignment was not done correctly, it was possible to damage the plastic of the unit.
Other switches that may be applicable for indicating tampering have an actuating plunger with a high surface kinetic friction, due to actuator shape, actuator finish and actuator material. However, the motion necessary to secure the body of the sensor to the mounting plate housing requires a clockwise rotation, and when the switch actuator contacts the activating cam on the housing there is heavy rotational stress due to the friction. The switch can be damaged as a result of the stresses introduced by the frictional shear force perpendicular to the switch's operating axis. The friction also makes the sensor hard to mount as it acts as an additional drag on the rotation.
Other sensor device designs counteract frictional stresses by using an actuating finger molded into the plastic. This finger rides up with a cam and produces a longitudinal force onto a switch's actuating plunger to assist in the switch closure. Unfortunately, depending on the design of this finger and the choice of materials, there could be a tendency for the plastic to cold-form over time and retain a permanent “set position”. This “set position” keeps the switch compressed when the sensor is disassembled.
It would therefore be desirable for a sensor device to signal tampering, and to be easy to install, and to simplify battery replenishment by an end-user. More specifically, it would be desirable for a sensor device to be mounted to a base without visually aligning any tabs or appurtenances and requiring a simple locking mechanism. It would further be desirable for a sensor device to have a tamper detection mechanism which would not be subject to “set positions” after a long period of time.